Solid state circuit components have brought incredible reduction in the size, weight and cost of audio amplifier circuitry and have also achieved increased fidelity in sound reproduction as compared with vacuum tube technology of a prior-generation. In an attempt to exploit to the limit the potential of solid state circuitry, audio engineers have striven to provide the user with increased power ratings while simultaneously achieving decreased distortion levels. Their efforts have met with resounding success, but have produced some undesirable side effects primarily in the areas of increased weight, cost and power consumption. For example, a commercially available state-of-the art 400 watt amplifier typically weighs anywhere from 16 kg to over 38 kg depending upon the particular design and choice of materials. Such amplifiers normally employ costly components necessitated by the peak loads which they must carry, and generate significant amounts of heat which must be dissipated to avoid component damage.
With regard to the transformer weight problem an obvious approach would be to reduce the number of windings and/or the gauge of the wire making up the transformer coils. However, reduction in the number of windings also reduces the inductance in the primary coil, thereby increasing idling currents through the coil and contributing to both heat generation and increased power consumption. The conventional method for achieving low idling currents in the primary has been to use a large number of windings. This approach also requires a large number of windings in the secondary to keep the voltage in the secondary at the proper level. The other obvious alternative for weight reduction, i.e. reduction in the wire gauge, is not an acceptable solution since the internal resistance of each coil would be increased, leading to excessive heat generation and power loss upon high power demands being placed on the transformer. Conventional wisdom has thus taught the necessity of increasing the size and weight of the transformer whenever a transformer powered amplifier is redesigned for increased power rating.
An alternative approach for reducing the overall weight, size and cost of audio amplifiers has been to reduce the total input power requirement without decreasing output power capability. Such increases in amplifier efficiency permit the use of less costly, lower weight power supplies, and can be achieved by reducing the power dissipation which normally attends the conventional use of output transistors in the output stage of the amplifier. When such power dissipation decreases are achieved, additional weight and cost savings are realized beyond those realized in the power supply since the weight, size and cost of the heat sinks normally required by the output transistors in the amplifier may also be reduced.
U.S. Pat. No. 3,426,290 to Jensen is representative of one known approach for increasing amplifier efficiency by keeping the voltage supplied to the output transistor of the amplifier very close to the output voltage level, thereby permitting operation of the output transistor in a condition which is at all times only slightly out of saturation. When operated in this condition, the actual voltage drop across the output transistor will be maintained quite low and the power dissipated by the transistor (equal to voltage across the transistor X current through the transistor) will be correspondingly reduced. A rather complex regulator is employed in the Jensen circuit to maintain the desired voltage supply to the output transistor wherein energy is stored in an inductive capacitive circuit by means of a switching transistor operated at high speed in response to a control signal derived from the audio input signal. By operating the switching transistor in full "on" or full "off" condition to maintain the desired voltage supply to the amplifier output transistor, energy consumption by the combined regulator and output transistor is reduced over that which would be consumed by an output transistor operated with a conventional fixed supply voltage. While producing a decided advantage in amplifier efficiency, the Jensen circuit is only truly effective if the switching transistor is operated at high frequencies, which can in turn cause transient interference distortion in the amplifier output signal. U.S. Pat. No. 4,054,843 to Hanada discloses a similar circuit to that disclosed in Jensen.
An alternative approach to achieving improved amplifier efficiency is disclosed in the patent to Dryden (U.S. Pat. No. 3,319,175) which discloses a stepped voltage supply operated in response to the voltage level of the amplifier output whereby the minimum voltage from the available power supply voltages sufficient to achieve the desired amplification is applied across the power amplifying element. While useful for the purposes disclosed, Dryden employs only a single transistor as the power amplifying element for each polarity of the output voltage and thus the entire difference between the load voltage and the connected supply voltage appears across the output transistor. Significant power losses will thus occur unless a large number of discrete supply voltages are provided by the power supply circuitry. Each such discrete voltage requires a separate amplitude comparator and associated switching device thus adding significantly to the cost of the power supply.
Still another approach disclosed in the prior art is illustrated in U.S. Pat. No. 3,622,899 to Elsenberg. In this patent a low power dissipation amplifier circuit is disclosed including plural transistors coupled in series to a load terminal wherein the transistors are energized by respective voltage sources having different magnitudes and wherein the transistors are biased to operate as amplifiers in sequence in response to an input signal of increasing magnitude. This type of circuit causes each output transistor to be driven into saturation as the next higher voltage output transistor is brought into operation, causing substantially the entire voltage drop in the amplifier output stage (that is the difference between the supply voltage and the load or output voltage) to appear across only a single output transistor at any one time. This arrangement of circuitry requires output transistors having substantial power ratings unless a relatively large number of output transistors and discrete supply voltages are provided. Either approach will add to amplifier cost. The patents to Woehner (U.S. Pat. No. 3,772,606) and to Sampei et al. (U.S. Pat. No. 3,961,280) disclose circuit arrangements similar to that described above with reference to the Elsenberg patent.
The patent to Schade, Jr. (U.S. Pat. No. 3,887,878) discloses a transistor series amplifier wherein plural series connected transistors in the output stage are biased to share the total voltage drop in the output stage to permit use of lower cost components. However, this patent fails to disclose a technique for reducing the total power dissipation in such transistors.
Still other techniques for reducing the cost of amplifier power supplies have been disclosed in the prior art. For example in the U.S. patent to Munch, Jr. (U.S. Pat. No. 3,542,953) a technique is disclosed wherein a single power supply may serve two Class B amplifier circuits designed to amplify the same audio signal by phase inverting the input to one amplifier to cause the amplifiers to draw peak current from the power supply in alternation. Munch, Jr. does not, however, suggest how such a technique can be employed in a system employing dual amplifiers (such as in a sterophonic system) for amplifying two separate signals.
None of the prior art systems discussed above addresses directly the problem of reducing power supply weight and costs by modifying the supply itself in a manner to employ less costly lighter, weight components while maintaining the power supply capabilities required by the amplifier circuit.
The patent to Chun (U.S. Pat. No. 3,466,527) discloses a circuit for reducing the cost and size of a transformer based voltage supply circuit including a duty cycle controlled switch in the A.C. power supply circuit of the transformer primary. The switch functions to regulate output voltage from the secondary. However, the lower cost and weight capability achieved by the concepts disclosed in Chun are derived by operating the duty cycle controlled switch over only a quarter cycle volt-time integral and do not in any way suggest how such a circuit design could be employed in an audio amplifier circuit in a manner to obtain power supply weight and cost reductions based on the characteristics of the incoming audio signals.